Pitch detector

ABSTRACT

The pitch of a complex speech wave is determined by spectrum analyzing the infinitely peak-clipped log spectrum of a centerclipped and infinitely peak-clipped interval of an analogue speech wave.

United States Patent [56] References Cited UNITED STATES PATENTS3,381,091 4/1968 Sondhi 179/1(AS) OTHER REFERENCES IEEE Transactions onAudio and Electroacoustics New Methods of Pitch Extraction, Man MohanSondhi, Vol. AU- 16, No. 2June 1968 Primary Examiner-Kathleen H. ClaffyAssistant Examiner-Jon Bradford Leaheey Att0rneysR. J. Guenther andWilliam L. Keefauver ABSTRACT: The pitch of a complex speech wave isdetermined by spectrum analyzing the infinitely peak-clipped logspectrum of a center-clipped and infinitely peak-clipped interval of ananalogue speech wave.

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m m- A k mmmw PITCH DETECTOR BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to the narrow band transmission ofspeech, and in particular to apparatus for identifying and analyzing thepitch of complex speech waves.

In the processing of complex speech waves it is often important todetermine whether a particular portion of such'wave is periodic oraperiodic and, if the wave is periodic, to determine its period orpitch. For example, in communications systems of the vocoder type onlyselected characteristics of a complex speech wave are transmitted to areceiving station which synthesizes an artificial replica of theoriginal speech signal. In such systems the voiced or unvoiced qualityof the speech signal (voiced speech being periodic, unvoiced beingaperiodic) and the pitch of the signal where voiced speech is presentare among the most important characteristics transmitted. It isparticularly important that pitch be determined accurately since smallerrors in pitch detection create a distorted and unnatural soundingoutput speech wave. An accurate wave analyzer for the purpose of voicepitch detection has been long sought.

2. Description of the Prior Art In a copending application Ser. No.420,362 filed Dec. 22, 1964 apparatus is described for determining thepitch of a complex wave such as a voiced speech wave by cepstrumanalysis. According to the spectrum method, which is thought to be asubstantial improvement over prior techniques, the square of the Fouriercosine transform of the logarithm of the power spectrum of a segment ofthe speech signal (defined as the cepstrum) is computed. The resultingspectrum signal is characterized by a peak at an interval proportionalto the fundamental pitch period during voiced or periodic portions ofthe speech signal, and by the absence of a peak during unvq iced oraperiodic portions of the signal. The theoretical considerationsunderlying spectral analysis and apparatus for carrying out suchanalysis are adequately described in the application referred to aboveand in an article entitled Spectrum Pitch Determination" in the Journalof the Acoustical Society of America, Vol. 41, No. 2, pp. 293309, Feb.1967 However, as the referenced application indicates, spectrum analysisrequires substantial signal processing involving repeated multiplicationoperations. If digital processing is employed, such multiplicationsrequire elaborate and expensive digital computing apparatus. It wouldthus be desirable to simplify the digital processing apparatus requiredto obtain the accuracy and reliability of spectrum pitch detection. Thiscould be accomplished ifthe numerous digital multiplications could beeliminated and replaced with simpler additive proceming.

Thus, it is an object of the present invention accurately to detect andanalyze the periodicity of a complex wave with comparatively simpleprocessing apparatus.

SUMMARY OF THE INVENTION In attaining this and other objects and inaccordance with the invention, a selected interval of a complex wave iscenterclipped, thus removing a selected fraction of the amplitude of thecomplex wave. The center-clipped signal is infinitely peakclipped andthe logarithm of the power spectrum of the resulting signal is computed.The clipped power spectrum is treated as a time varying signal and assuch is low-pass filtered to remove DC and selected low frequencycomponents and again infinitely peak-clipped. The square of the Fouriercosine transform of the resulting signal, which is defined as theclipstrum of the speech signal, is then examined. The presence of peaksin this function indicates the presence of periodic waves in the inputsignal. Where periodic waves are present, the location of peaks in theclipstrum establish the period of such waves.

BRIEF DESCRIPTION OF THE DRAWING The invention will be fully apprehendedfrom the following description of an illustrative embodiment thereoftaken in conjunction with the appended drawings wherein:

FIG. 1 is a block diagram illustrating a vocoder transmission systememploying a pitch analyzer constructed in accordance with the invention;

FIG. 2 is a block schematic drawing of a pitch analyzer constructed inaccordance with the invention and suitable for inclusion in a system ofthe type shown in FIG. 1;

FIG. 3A illustrates a typical speech waveform;

FIG. 3B is a center-clipped speech waveform;

FIG. 3C depicts a centerclipped and infinite peak-clipped speechwaveform; a

FIG. 3D shows a typical frequency spectrum of a centerclipped andinfinitely peak-clipped speech waveform;

FIG. 3E is the waveform of FIG. 3D after low frequency and DC componentsare filtered out;

FIG. 3F is the waveform of FIG. 3E after infinite peakclipping; and

FIG. 3G illustrates the clipstrum of a speech waveform.

DETAILED DESCRIPTION The vocoder system shown in FIG. 1 can be employedin the transmission of speech or other complex signals over narrow bandcommunications channels. The system includes a transducer, 10, forconverting an acoustic speech wave into an electrical analogue wavewhich occupies a frequency band approximately equivalent to the speechband. Direct transmission of such an analogue signal would normallyrequire a transmission channel capable of accommodating the normalspeech frequency band. The vocoder apparatus shown in FIG. 1 permitstransmission of such a voice band signal over a narrow bandcommunications channel by converting the analogue speech signal at thetransmitting station 15 into a relatively small number of low frequencycontrol signals which together occupy only a small frequency band butwhich adequately describe the speech signal. A replica of the speechsignal is then synthesized at the receiving station 16 from thesecontrol signals. 7

The vocoder system shown in FIG. 1 includes, at the transmitting station15, a vocoder analyzer 11 which may for example be a conventionalchannel vocoder analyzer of the type shown in H. W. Dudley U.S. PAT. No.2,151,091 issued Mar. 21, I939 Such analyzer derives from an inputspeech wave a group of narrow band control signals representing in codedform the energy within each of a number of selected frequency subbandsof the speech signal. Also at the transmitting station, a pitch detector12 to be described in detail below and a pitch encoder l7 derive andcode a pitch control signal which indicates the presence and pitch ofvoiced speech.

The control signals generated by the vocoder analyzer and the pitchcoder are transmitted over a limited bandwidth communications channel toreceiving station 16. The receiver includes a vocoder synthesizer 13 anda pitch decoder 18 in conjunction with an excitation generator 14. Thepitch decoder and excitation generator receive the coded pitch signaland generate an excitation wave with a fundamental frequency equivalentto the fundamental frequency of voiced speech in the original speechsignal. The excitation signal is applied to the vocoder synthesizerwhich also receives the subband control signals and reconstructs areplica of the original speech wave by combining the excitation'wavewith the subband control signals. The vocoder synthesizer may be aconventional channel synthesizer of the type described in the abovereferenced Dudley patent and the pitch coder, pitch decoder andexcitation generator may be of a type described in H. S. McDonald U.S.Pat. No. 3,190,142 issued Oct. 29, 1963.

it has been found that the quality of the speech signal reproduced bythe vocoder apparatus described above is largely dependent on theaccuracy of the pitch detector employed. Thus, in accordance withtheinvention, such pitch informationisprovided by a pitch detector oftheitype shown in detail in FIG. .2.

Referring to FIG. 2, speechdetector I2 is designed to analyzeaconventional analogue speech signal or other com- "plex wave and toindicate the existence of voiced orperiodic of smaller amplitude may beproduced 'at intervals equal to multiples .of the fundamental period,but these can be disregardedl .When no periodic energy is present, nospike occurs. The output signal of detector 12 may thus be applied to anencoding network such as pitch encoder 17 in FIG. 1 which interprets thespike location and produces an appropriate control signal forcommunicating the pitch of a periodic input signal to a receivingdevicefl'his network may include apparatus for detecting peaks in asignal such as that described in .a copending application Ser. No.508,726 filed Nov. 19, I965 by A. M. Noll now U.S. Pat. No. 3,420,955When the input signal is notperiodic, this fact too may be communicatedto the receiver.

Thus, in FIG. 2, the input signal applied to channel 20 may be a segmentof a conventional analogue speech signal prepared ina' manner well knownin the speech processing art.

Such a signal is shown in FIG. 3A. The input signal is applied tocenter-clipper 21, of any construction well known in the electronicarts, which has theeffect of removing a center segment of the amplitudeof the input wave at a selected level above and below thezero axis. Thesignal levels marked U" for upper threshold and L'for lower threshold inFIG. 3A denote one'possible range of center-clipping. FIG. 3B shows thesignal of FIG. 3A center-clipped betweenU and L. It is to be understoodthat the actual threshold levels U and L for .centerclipper 21 may varyfrom signalsegrnent to signal segclipping on the order of 70 percent ofthe absolute maximum of the input wave in each interval is mosteffective in analyzing a common speech signal.Center-clippingcircuitssuitable for use in thenetwork shown in'FlG. 2are described by M. M. Sondhi in US. Pat. No. 3,381,091 issued Apr. 30,I968.

The center-clipped output of network 2l'is applied to infinitepeak-clipper 22. Peak-clipping is analogous to centerclippingexceptthat, rather than eliminating the central amplitude portion of theinput wave as centerclipping does, a peak-clipper eliminates the extremehigh and 'low amplitude portions of the wave, leaving onlytheintermediate section. Infinite peak-clipping removes the entire wavestructure except for that occurring in the immediate vicinity of thezero axis. Ineffect, only the zero crossing information is retainedafter infinite peak-clipping. One possible output of infinitepeak-clipper 22 is shown in FIG. 3C. This signal has an arbitraryamplitude selected for convenience zero crossings directly related tothe zero crossings in the waveform shown in FIG. 38. It is to beunderstood that other waveforms which maintain only zero crossinginformation could be produced by infinite peak-clipper 22. Infinitepeak-clipping networks suitable for inclusion in the system shown inFIG. 2 are well known in the electronic arts.

The infinitely peak-clipped output of network 22 is applied to logspectrum analyzer 23. Analyzer 23 produces a signal of the form shown inFIG. 3D which represents the amplitude of the various frequencycomponents of the applied signal plotted versus frequency. It isobserved in FIG. 31) that the spectrum has the appearance of a waveformcharacterized by a fine wave structure superimposed upon a coarse wavestructure. In the case where the wave applied to analyzer I2 is a speechwave, the long wavelength peaks in FIG. 3D represent remnants of theformant structure of the initial wave and other disturbances and theperiod of the short wavelength peaks represents'the fundamentalfrequency of the incoming speech wave.

Log spectrum analyzer 23 may be any one of numerous spectrum analyzingdevices. It may be an analogue heterodyne spectrum analyzer of the typedescribed'in the aforementioned copending application filed Dec.'22,1964,'Ser. No. 420,362 or may be similar to the analyzer described by M.R. Schroeder in US. Pat. No.-3,32l,582 It is to be noted that theanalyzer may be analogue or a digital device. If a-digital spectrumanalyzer is employed; the signal processing is very Whatever form ofspectrum analyzer is employed, the

analyzer output is applied to signal adjusting network 24 wherein the DCand low frequency variations in theanalyzer output signal are removed.If the signal resulting from analyzer 23 is treated as a timevaryingsignal, this process is in effect a filtering process in the timedomain. Since the output of analyzer 23 may be either in digital oranalogue form, the adjusting apparatus of network 24 is selectedaccordingly. Digital or analogue filters suitable for performing thisadjusting function are well known in the signal processing art.

The output of adjustingnetwork 24, shown in FIG. 3E is treated as a timevarying signal and is applied to infinite peakclipper 25 which issimilar in design and operation to infinite peak-clipper 22 describedabove. The output waveform of clipper 25 takes the form of a square waveas 'shown in FIG. 3F. It will be seen that the output of clipper 25,shown in FIG. SP, is more regularly periodic than the'initial' speechwave shown in FIG. 3A.This regular periodicity, which is related to thefundamental period of the voiced speech elements in the initial speechsignal, is detected by spectrum analyzer 26 which is similar to analyzer23 described above. The output of analyzer 26, which appears in channel27 contains a spike as shown in FIG. 3G at a time proportional to thefundamental period of the voiced components of the input speech wave.Smaller amplitude spikes representing higher order harmonics mayalsoappear'. If the input signal does not contain voiced or periodicspeech elements, the high frequency component in the output of network23, shown in FIG. 3D, will not exist and no spike will appear in theoutput signal from analyzer 26. Thus the absence of a spike in thesignal output from network 26 can be taken to indicate the absence ofvoiced speech in the input wave.

It is to be understood that the above-described arrangements are merelyillustrative of the invention. Other arrangements may be devised bythose skilled in the art without departing from the spirit and scope ofthe invention.

I claim:

1. Apparatus comprising, in combination;

means for center-clipping a complex wave,

means supplied with signals from said center-clipping means forinfinitely peak-clipping said center-clipped complex wave;

first means for developing a signal representative of the logarithm ofthe power spectrum of said infinitely peakclipped complex wave;

means for removing selected low-frequency variations in said spectrumrepresentative signals to produce an adjusted spectrum, signal;

means for infinitely peak-clipping said adjusted signal; and

second means for developing an output signal representative of the powerspectrum of said infinitely peak-clipped adjusted signals.

2.,Apparatus as defined in claim 1 further including means foridentifying the existence of peaks in said output signal.

3. Apparatus as defined in claim 2 further including means for measuringthe location on the time axis of peaks in said output signal.

4. Apparatus as defined in clam 3 wherein said first and second formeans for developing spectrum signal are heterodyne spectrum analyzers.

5. Apparatus as defined in claim 3 wherein said first and secondanalyzer means are digitized Fourier transform analyzers.

6. Apparatus which comprises, in combination;

a source of complex wave signals;

a center-clipping nework supplied with said complex wave signals;

a first infinite peak-clipping network supplied with signals from saidcenter-clipping network;

a log spectrum analyzer network for analyzing the frequency componentsof signals produced by said infinite peakclipping network;

a signal adjusting network for removing the slow variations in theoutput from said first spectrum analyzer;

a second infinite peak-clipping network supplied with signals from saidsignal adjusting network; and

a second spectrum analyzing network for analyzing the frequencycomponents of signals from said second infinite peak-clipping network.

7. Apparatus for analyzing the periodicity of a complex wave whichcomprises:

means for removing a selected central portion of the amplitude of aselected interval of said complex wave;

means for generating a first square wave signal with axis crossingsrelated to the axis crossings in the signal produced by said removingmeans;

means for generating a signal related to the power spectrum of saidsquare wave signal;

A means for producing a logarithm signal proportional to the logarithmof said power spectrum signal;

means for removing the slow variations of said logarithm signal;

means for generating a second square wave signal with axis crossingsrelated to the axis crossings of said logarithm signal; and

means for generating a signal proportional to the power spectrum of saidsecond square wave signal.

1. Apparatus comprising, in combination; means for center-clipping acomplex wave, means supplied with signals from said center-clippingmeans for infinitely peak-clipping said center-clipped complex wave;first means for developing a signal representative of the logarithm ofthe power spectrum of said infinitely peak-clipped complex wave; meansfor removing selected low-frequency variations in said spectrumrepresentative signals to produce an adjusted spectrum, signal; meansfor infinitely peak-clipping said adjusted signal; and second means fordeveloping an output signal representative of the power spectrum of saidinfinitely peak-clipped adjusted signals.
 2. Apparatus as defined inclaim 1 further including means for identifying the existence of peaksin said output signal.
 3. Apparatus as defined in claim 2 furtherincluding means for measuring the location on the time axis of peaks insaid output signal.
 4. Apparatus as defined in clam 3 wherein said firstand second for means for developing spectrum signal are heterodynespectrum analyzers.
 5. Apparatus as defined in claim 3 wherein saidfirst and second analyzer means are digitized Fourier transformanalyzers.
 6. Apparatus which comprises, in combination; a source ofcomplex wave signals; a center-clipping network supplied with saidcomplex wave signals; a first infinite peak-clipping network suppliedwith signals from said center-clipping network; a log spectrum analyzernetwork for analyzing the frequency components of signals produced bysaid infinite peak-clipping network; a signal adjusting network forremoving the slow variations in the output from said first spectrumanalyzer; a second infinite peak-clipping network supplied with signalsfrom said signal adjusting network; and a second spectrum analyzingnetwork for analyzing the frequency components of signals from saidsecond infinite peak-clipping network.
 7. Apparatus for analyzing theperiodicity of a complex wave which comprises: means for removing aselected central portion of the amplitude of a selected interval of saidcomplex wave; means for generating a first square wave signal with axiscrossings related to the axis crossings in the signal produced by saidremoving means; means for generating a signal related to the powerspectrum of said square wave signal; A means for producing a logarithmsignal proportional to the logarithm of said power spectrum signal;means for removing the slow variations of said logarithm signal; meansfor generating a second square wave signal with axis crossings relatedto the axis crossings of said logarithm signal; and means for generatinga signal proportional to the power spectrum of said second square wavesignal.